Attachment of Stress Sensitive Integrated Circuit Dies

ABSTRACT

In an embodiment, a semiconductor package includes a support and a stack of two or more semiconductor dies, the stack including an upper die and further including a lower die attached to the support by adhesive on a backside of the lower die, wherein the adhesive covers only part of the backside of the lower die, and wherein the adhesive has a plurality of non-contiguous regions on the backside of the lower die.

This is a divisional patent application of U.S. Application No.16/759,659, entitled “Attachment of Stress Sensitive Integrated CircuitDies,” which was filed on Apr. 27, 2020, which is a national phasefiling under section 371 of PCT/EP2018/081569, filed Nov. 16, 2018,which claims the priority of U.S. Pat. No. 62/587,511, filed Nov. 17,2017, each of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to the attachment of an integratedcircuit die to a carrier.

BACKGROUND

Pressure sensors, such as microelectromechanical systems (MEMS) sensors,have many applications. These sensors can be used, for example, inautomotive, consumer, industrial, medical, and other applications. InMEMS sensors, for example, pressure can be measured via deflection of amembrane caused by an external pressure. Large deflections ortemperature differences, however, can induce significant non-linearityin the sensors, which may present challenges in a variety ofapplications. Accurate and repeatable manufacturing processes ofmembranes and pressure sensors can allow for more accurate pressurereadings over a range of temperatures and pressures.

Although some thermal effects and related stresses are predictable andthus can be included in calibrated devices, the overall stress state ofthe sensor die may be altered by other influences, such as bending ofthe carrier on which the sensor is mounted and/or moisture uptakeleading to non-uniform swell of the carrier. For an ultra-sensitivepressure sensor, such changes often lead to undesirable sensor outputdrift.

SUMMARY

Embodiments provide packages that can house, for example, a stresssensitive die that needs to be packaged in a low profile package forwearable/consumer/mobile markets and that can benefit from stressdecoupling without increasing the build-up height. In general, thepackage includes a semiconductor die attached to a support by adhesiveon a backside of the die. The adhesive covers only part of the backsideof the die and can be formed, for example, as stripe-shaped or othernon-contiguous regions on the backside of the die.

For example, in one aspect, the present disclosure describes asemiconductor package that includes a support, and a die attached to thesupport by adhesive on a backside of the die. The die includes acapacitive pressure sensor integrated on a CMOS read-out circuit. Theadhesive covers only part of the backside of the die.

Some implementations include one or more of the following features. Forexample, the adhesive can have multiple non-contiguous regions on thebackside of the die. In some instances, the adhesive has twonon-contiguous stripe-shaped regions on the backside of the die. Thestripe-shaped regions of adhesive can be disposed, for example, adjacentedges of the die. In some cases, e.g., where the capacitive pressuresensor includes a rectangular, suspended tensile membrane, thestripe-shaped regions of adhesive can be oriented parallel to the longersides of the membrane.

The present disclosure can be particularly advantageous forimplementations in which the die has a maximum thickness no greater than250 um and/or a packaged product total height no greater than 0.8 mm.

In another aspect, the present disclosure describes a semiconductorpackage including a support, and a die attached to the support byadhesive on a backside of the die. The adhesive covers only part of thebackside of the die and has multiple non-contiguous (e.g.,stripe-shaped) regions on the backside of the die.

In yet a further aspect, the present disclosure describes asemiconductor package including a support, and a stack of two or moresemiconductor dies. The stack includes an upper die and a lower die. Insome cases, the lower die is attached to the support by adhesive on abackside of the lower die such that the adhesive covers only part of thebackside of the lower die, and has multiple non-contiguous regions onthe backside of the lower die. In some cases, the upper die is attachedto the lower die by adhesive on a backside of the upper die such thatthe adhesive covers only part of the backside of theupper die, andwherein the adhesive has multiple non-contiguous regions on the backsideof the upper die.

Some implementations include one or more of the following advantages. Insome cases, patterned adhesive improves the package compared to using asolid layer of adhesive. For example, adhesive can act as a rollerbearing, preventing bending moments from being transferred to thepressure sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features and advantages will be readily apparent from thefollowing detailed description, the accompanying drawings and theclaims.

FIG. 1 illustrates an example of a package housing a semiconductor die;

FIG. 2 illustrates an example of a die including capacitive pressuresensor;

FIG. 3A illustrates a first example of an adhesive pattern. FIG. 3B is atop view of the arrangement of FIG. 3A;

FIG. 3C illustrates a second example of an adhesive pattern. FIG. 3D isa top view of the arrangement of FIG. 3C;

FIGS. 4A and 4B illustrate an example of a stack of dies. FIG. 4B is atop view of the arrangement of FIG. 4A;

FIGS. 5A and 5B illustrate an example of a stack of dies. FIG. 5B is atop view of the arrangement of FIG. 5A;

FIG. 5C is a top view for an alternative implementation of FIG. 5A inwhich the membrane of the capacitive sensor is square shaped; and

FIG. 6 illustrates an example for a microphone/pressure sensorcombination package.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As shown in FIG. 1 , a device package 10 includes a die (e.g.,semiconductor chip) 12. In the illustrated example, the die 12 includesan application specific integrated circuit (ASIC) with an integratedcapacitive pressure (e.g., MEMS) sensor. The package 10 includes asupport 14 to which the die 12 is attached. The support 14 can beimplemented, for example, as a single or multi-layered (e.g., laminated)substrate, whose surface facing the interior of the package 10 mayinclude a die pad 16 and one or more bond pads 18 for electricalconnections 20 to and from the integrated circuit die 12. The die 12 isattached to the support (e.g., the die pad) 16 by an adhesive 22 (e.g.,glue) present on only a portion of the backside of the die 12. Thus, inareas the die’s backside where no adhesive is present, there is a smallgap between the upper surface of the die pad 16 and the backside of thedie 12.

The package 10 also includes one or more solder pads 24 on its outerlower surface. The package 10 further includes a metal or other cap 26that shields the die 12. The cap 26 can have a small opening 28 thatprovides access to ambient pressure. In some implementations, the cap 26is fully closed, but the support 14 has a port to provide access toambient pressure.

In some implementations, the package 10 is relatively thin (e.g., < 8mm) and incorporates a single die 12 that has a thickness no greaterthan 250 um. In some cases, the height of the package is less than 0.7mm.

FIG. 2 illustrates various details of an example of a capacitivepressure sensor that can be integrated into the die 12. As shown in FIG.2 , a semiconductor device 100 includes a capacitive pressure sensor 108formed over an integrated circuit 106. The capacitive pressure sensor108 includes a suspended tensile membrane 102 over a cavity 112. Thesensor 108 also includes a bottom electrode 104, which in someimplementations is formed on top of the final passivation layer of aCMOS read-out circuit. The electrode(s) and suspended membrane of thecapacitive pressure sensor 108 can be connected electrically to theintegrated circuit 106. The bottom electrode 104 may be segmented andmay include multiple annular rings. Two or more anchor trenches 11laterally surround the cavity 112, are filled with a first electricallyconductive material, and are separated from one another by an oxidesupport layer (e.g., silicon oxide) 126.

The first electrically conductive material that fills the anchortrenches 114 can include, for example, a PVD Ti/TiN liner and CVDtungsten (W). The cavity 112 sidewalls are formed, at least in part, bythe conductive material of the inner anchor trench 114A. The suspendedmembrane 102 can be composed of a second electrically conductivematerial (e.g., tungsten (W)) and extends beyond the outer anchor trench114B. The first electrically conductive material 114 thus serves assupporting anchors for the suspended membrane 102. The firstelectrically conductive material 114 and the membrane 102 form part of atop electrode that is suspended above the bottom electrode 104. Thecavity 112 separates the membrane 102 and bottom electrode 104 from oneanother. An isolation trench 130 can separate the bottom electrode fromconnections 120 for the top electrode. The semiconductor device 100 alsodepicts electrically conductive connections 120 to connect the topelectrode or the membrane 102 to the integrated circuit 106 orelsewhere. The semiconductor device 100 also may include aluminum orother contact pads to provide connections to another device. Variousvias may extend down from the contact pads to the bottom electrode, andalso from the bottom electrode to the CMOS top metal layer.

The foregoing details, illustrated and described in connection with FIG.2 , are simply an example of the type of die 12 that can be attached byadhesive 22 to the die pad 16 in the package 10. Thus, the variousinventive concepts described in this disclosure can be used with otherintegrated circuit dies as well.

As mentioned above, the die 12 is attached to the die pad 16 by anadhesive 22 present on only a portion of the backside of the die 12.This can be accomplished, for example, by depositing the adhesive 22 ina pattern on selected areas of the backside multiple (e.g., two, three,four or more) areas of the backside of the die 12. For example, as shownin FIGS. 3A and 3B, a small amount of adhesive 202 is provided on thebackside of the die 12 at its four comers 204. In some cases, theadhesive dots 202 have a diameter on the order of about 300 pm, 200 pm,or even less. Other areas of the die’s backside are not covered withadhesive. Deposition of the adhesive at the comers 204 of the die 12results in suspension of the die 12 at the comers and can provide areduction in the transfer of bending moments to the suspended die 12 intwo directions. In the example of FIGS. 3A-3B, the integrated pressuresensor includes two membranes 102A, 102B. The number of membranes maydiffer for other implementations.

In some implementations, the adhesive 22 is provided in the form of dotson multiple (e.g., two, three, four or more) areas of the backside ofthe die 12. For example, as shown in FIGS. 3A and 3B , a small amount ofadhesive 202 is provided on the backside of the die 12 at its fourcomers 204. In some cases, the adhesive dots 202 have a diameter on theorder of about 300 pm, 200 pm, or even less. Other areas of the die’sbackside are not covered with adhesive. Deposition of the adhesive atthe comers 204 of the die 12 results in suspension of the die 12 at thecomers and can provide a reduction in the transfer of bending moments tothe suspended die 12 in two directions. In the example of FIGS. 3A-3B,the integrated pressure sensor includes two membranes 102A, 102B. Thenumber of membranes may differ for other implementations.

In some implementations, as shown in FIGS. 3C and 3D, the adhesive isprovided in the form of multiple stripes 206 on the backside of the die12. The adhesive stripes 204 can be provided, for example, near edges208 of the die 12. In some instances, such as for a rectangular-shapedpressure sensor membrane 102, the stripes 204 of adhesive in aparticular orientation with respect to the orientation of the membrane102 on the die 12. For example, in the illustrated example, in which thepressure sensor includes two membranes 102A, 102B, the adhesive stripes204 are oriented parallel to the longer sides of the rectangularmembranes. Such patterning of the adhesive can help reduce the bendingmoments in at least one direction (i.e., perpendicular to the directionof the adhesive stripes 204). As rectangular membranes generally aremore stress sensitive in the direction parallel to the membranes’respective shorter sides, providing the adhesive on the backside of thedie 12 in a direction parallel to the membranes’ longer sides can beadvantageous by helping reduce the stress. The number of adhesivestripes may differ in some implementations. Likewise, adhesive stripescan be used when the pressure sensor includes a different number ofmembranes.

Various adhesives may be used. In some instances, a flexible adhesivehaving a Shore Durometer Hardness level (Shore A) below 50 is used. Insome cases, it is desirable to use a low-stiffness silicone-basedadhesive (e.g., Semicosil® 988/lk adhesive available from Wacker ChemieAG). In some instances, silicone-based glues and B-stage glues with lowYoung’s modulus can be used. Some of the adhesives are heat-curable, andin some cases, are cured at elevated temperatures (e.g., in atemperature range of 100° C. - 200° C.) rather than at room temperature.In some instances, the adhesive can be, or include, polydimethylsiloxane(PDMS). For some implementations, the adhesive can be based on acrylatechemistries or a polycarbamin acid derivative. The latter can beadvantageous because partial curing can be accomplished by exposure toultra-violet (UV) radiation, followed by die placement and final thermalcuring. Thus, the shape of the adhesive deposits can be retained moreeasily (i.e., not adversely affected by flow during die placement andcuring). Such adhesives are available, for example, from DELO IndustrialAdhesives of Germany (e.g., DELO DUALBOND® AD345).

In some implementations, the adhesives can be dispensed easily using anyof a wide range of dispensing equipment. The adhesive can be dispensed,for example, from a nozzle. The adhesive can be applied, for example, soas to provide mechanical confinement, e.g., by means of a rim on top ofthe support 14 or recess in the support. For some implementations (e.g.,very small dies), an adhesive should be selected such that the adhesivecan be dispensed as small glue droplets with sufficient stand-offheight. If the adhesive has too low thixotropy (i.e., shear thinningeffect), the adhesive may flow too easily, thereby destroying itsdesired shape. In some instances, the silicone adhesive can be appliedat least two times to increase adhesive height and to avoid adhesiveoutflow.

By separating the adhesive layer into separate regions, variousadvantages can be obtained in some implementations. In some cases, theadhesive layer reduces the ability of deformations to be transferredfrom the substrate 14 to the die 12. Also, an air channel can beprovided in the adhesive to allow rapid in and out diffusion of watervapor. The contact areas for the adhesive 22 and the substrate 14 to theambient are effectively increased, thereby reducing diffusion times foroxygen, water vapor or other gases that might act on the polymers of theadhesive 22 and/or substrate 14. Reduction of these times may reducedelay in temperature-dependent sensor response.

The use of patterned adhesive 22 can result in a packaged sensor inwhich bending moments originating from mechanical deformation orhygroscopic swelling are eliminated or significantly reduced. Further,the foregoing techniques can be particularly advantageous for thinpackages (i.e., < 0 8mm) that incorporate a single die that has athickness no greater than 250 um. In particular, the techniquesdescribed here can improve stress decoupling without increasing theoverall build-up height of the package 10.

The techniques described here can, in some instances, provide a low-costsolution that improves the accuracy of a pressure sensor. The techniquescan enable accurate usage of the sensor in environments of non-constantrelative humidity of the ambient air. Such features can enhance usage ofthe sensor for applications relating to indoor navigation, such as whereone enters an air-conditioned shopping mall from outside where it ishumid. Even in such situations, the barometric height as calculated bythe pressure sensor should remain stable.

In addition, the techniques described here can enable maintenance ofhigh accuracy even for conditions in which the temperature isnon-constant. As is known, due to differences in the coefficient ofthermal expansion (CTE), varying temperatures may cause varying levelsof board- and package-level stresses. The enhanced level of stressdecoupling using patterned adhesive can eliminate or reduce thesestresses.

Although the techniques described here can be particularly advantageousfor a package housing a single die 12 that includes an ASIC with anintegrated capacitive pressure sensor, the techniques also can be usedfor situations in which the package houses two or more semiconductordies stacked one atop the other (e.g., a sensor due stacked on a CMOSread-out circuit die). In some cases, as shown in FIGS. 4A and 4B, thelower die (e.g., a CMOS read-out circuit die) 12B is attached, forexample, to the die pad of the support 14 using adhesive applied to thebackside of the lower die such that the adhesive covers only part of thebackside of the lower die 12B (e.g., using adhesive dots 202 at thecomers of the die’s underside as shown in FIGS. 4A-4B, or using stripesof adhesive). The upper die (e.g., the die including the stresssensitive sensor or transducer) 12A can be attached to the lower die12B, for example, by a standard die-attach foil 30. Electricalconnections 20A can be provided between the upper and lower dies 12A,12B, for example, using bond wires.

In some cases, as shown in FIGS. 5A and 5B, the lower or bottom die(e.g., the CMOS read-out circuit die) 12B is attached to the die pad ofthe support 14 using a standard die-attach foil 30. The upper or top die(e.g., the die including the stress sensitive sensor or transducer) 12A,however, can be attached to the bottom die 12B using adhesive 22 appliedto the backside of the top die 12A such that the adhesive covers onlypart of the backside of the upper die 12A (e.g., using adhesive dots s02at the comers of the die’s underside as shown in FIGS. 5-4B, or usingstripes of adhesive). Although the illustrated example of FIG. 5B showsa rectangular membrane 102 having shorter and longer sides, in somecases, the membrane 102 can be square shaped as shown in FIG. 5C.Further, some implementations may include more than one membrane.

The arrangement of FIGS. 5A-5B can be used, for example, for amicrophone/pressure sensor combination package in which the bottom die12B is an ASIC (e.g., a CMOS read-out circuit) for the microphone, andthe top die 12A is an ASIC that includes an integrated pressure sensorincluding one or more membranes 102A, 102B. FIG. 6 illustrates such animplementation that includes electrical connections 20, 20A from theintegrated pressure sensor die 12A and the CMOS read-out circuit die 12Bto respective bonding pads 18 on the support 14, and that furtherincludes electrical connections 20B from the CMOS read-out circuit die12B to a microphone die 12C.

Each of the foregoing implementations discussed in connection with FIGS.3A-3D, 4A-4B, 5A-5C and 6 can form part of a package that houses thedie(s) as shown, for example, in FIG. 1 .

Other implementations are within the scope of the claims.

1. A semiconductor package comprising: a support; and a stack of two ormore semiconductor dies, the stack including an upper die and furtherincluding a lower die attached to the support by adhesive on a backsideof the lower die, wherein the adhesive covers only part of the backsideof the lower die, and wherein the adhesive has a plurality ofnon-contiguous regions on the backside of the lower die.
 2. The packageof claim 1, wherein the upper die includes a capacitive pressure sensor,and wherein the lower die includes a CMOS read-out circuit.
 3. Asemiconductor package comprising: a support; and a stack of two or moresemiconductor dies, the stack including a lower die attached to thesupport and further including an upper die attached to the lower die byadhesive on a backside of the upper die, wherein the adhesive coversonly part of the backside of the upper die, and wherein the adhesive hasa plurality of non-contiguous regions on the backside of the upper die.4. The package of claim 3, wherein the upper die includes a capacitivepressure sensor, and wherein the lower die includes a CMOS read-outcircuit.
 5. A semiconductor package comprising: a support; and a dieattached to the support by adhesive on a backside of the die, whereinthe adhesive covers only part of the backside of the die, the adhesivehaving two or a plurality of non-contiguous stripe-shaped regions on thebackside of the die.
 6. The package of claim 5, wherein thestripe-shaped regions of the adhesive are disposed adjacent edges of thedie.
 7. The package of claim 6, wherein the die includes a capacitivepressure sensor having a rectangular, suspended tensile membrane, andwherein the stripe-shaped regions of adhesive are oriented parallel tolonger sides of the membrane.
 8. The package of claim 5, wherein the diehas a maximum thickness no greater than 250 µm.
 9. The package of claim5, wherein the package has a maximum height no greater than 0.8 mm. 10.The package of claim 5, further comprising an air channel in theadhesive, the air channel configured to allow out diffusion of watervapor.
 11. The package of claim 5, wherein the adhesive includes aplurality of non-contiguous regions on the backside of the die at itscorners.
 12. The package of claim 5, wherein the support includes a diepad, and wherein the die is attached by the adhesive to the die pad.